Hitherto, various plastic sheets with folding rules have been known, for example, a plastic sheet with folding rules forming folding rules composed of shallow grooves and deep grooves in folding portions by pressing a rule cutter having a concave blade and convex blade formed on a blade tip portion (Japanese Utility Model Publication No. H4-9345), and a plastic sheet with folding rules forming folding rules composed of semicircular shallow grooves as seen from the side and straight deep grooves in folding portions by pressing a rule cutter having a semicircular concave blade as seen from the side and straight convex blade formed on a blade tip portion (Japanese Laid-open Patent No. 2001-293777).
However, in these plastic sheets with folding rules, when the bottom thickness of folding rules provided by the rule cutter is reduced, the folding characteristic of the plastic sheet is enhanced, but if the thin portion is continuous in the longitudinal direction, when the plastic sheet is folded along the folding rule, cracks or large holes may be likely to be formed in the bottom of the folding rule. If the rule cutter is pressed firmly against the plastic sheet and the thin portion of the bottom of the folding rule is opened intentionally, dust, moisture or other foreign matter may invade from this portion, and the commercial value or sanitation of the commodities contained in the packaging container may be spoiled. At the same time, the groove step difference is large and corners are formed at acute angle, hand feeling of the folding portion is poor, and cloth or skin may be caught. Or when cutting deeply by the rule cutter and the groove thickness is reduced, the plastic sheet is spread in the surface direction and the molecular density is raised. A sheet of small tensile strength or elongation may be torn at the folding rule, and biodegradable plastic sheet of small elongation may not be formed. In addition, distortion, warping or corrugation may occur in the folding portion, and the appearance quality is poor. When assembling a plastic sheet into a packaging container by a box making machine, it may be stagnant or clogged, and it is hard to assemble mechanically.
The invention is devised in the light of the above problems, and it is hence an object thereof to present a plastic sheet with folding rules and rule cutter for plastic sheet capable of obtaining favorable folding characteristic and flexibility by providing a plastic sheet with folding rules forming a plurality of deep grooves in the bottom between shallow grooves, thereby improving the hand touch and appearance, suppressing occurrence of warp or corrugation, and folding mechanically.